A fiber laser device has been widely used in such applications as inscription of a character on a metal plate and microfabrication of metal. A fiber laser device includes (i) an amplification fiber having a core to which rare earth has been added and (ii) a laser diode for emitting excitation light. A fiber laser device outputs laser light by exciting the amplification fiber with excitation light.
A fiber laser device includes a laser diode driving device for driving the laser diode. Patent Literatures 1 to 3 each disclose a light source driving device for driving a semiconductor light source such as a laser diode and a light-emitting diode.
Patent Literature 1 discloses a light-emitting diode driving device that is capable of causing a constant current to flow through a plurality of light-emitting diodes regardless of whether the output voltage of a direct-current power supply used is higher or lower than the total of respective forward voltages of the light-emitting diodes. Patent Literature 1 further takes into consideration such aspects as a temperature-dependent change in the forward voltages and variation among the respective forward voltages of the individual light-emitting diodes. Patent Literature 2 discloses a light-emitting diode driving device that carries out feedback control from the output side in order to adjust respective amounts of electric power to be supplied to a plurality of light-emitting diodes.
Further, there has been developed a fiber laser device that is capable of automatically controlling turning on and off of laser light output in correspondence with a processing order. Such a fiber laser device is capable of not only an operation that keeps steadily outputted laser light ON, but also an operation that turns laser light ON and OFF in a certain cycle (several hundreds of microseconds or longer). The fiber laser device is further capable of changing its laser light output for each period during which the light output is ON.
FIG. 8 is a block diagram schematically illustrating a configuration of a laser diode driving device 101 included in a conventional fiber laser device. The laser diode driving device 101 is a device for driving n laser diodes LD1 to LDn connected to one another in series, and includes a DC power supply 102, a current driving element 103, a current control section 104, and a fiber laser control section 105.
The DC power supply 102 is a constant voltage source, and outputs a voltage Vcc for driving the laser diodes LD1 to LDn. The current driving element 103 includes, for example, an N-channel type MOS transistor, and causes a current If to flow through the laser diodes LD1 to LDn.
The fiber laser control section 105 supplies, on the basis of a preset sequence program, the current control section 104 with a sequence control signal indicative of a light output P and output continuing period t for the laser diodes LD1 to LDn. The current control section 104 outputs, on the basis of the sequence control signal, a current control signal to the gate terminal of the current driving element 103 to control (i) turning ON and OFF of the current driving element 103 and (ii) the amount of the current If.
FIG. 8 shows Rall, which indicates resistance in the circuit constituted by the DC power supply 102, the current driving element 103, and the laser diodes LD1 to LDn which resistance is of wiring and the like excluding the current driving element 103 and the laser diodes LD1 to LDn.
(a) through (g) of FIG. 9 illustrate respective waveforms of, for example, (i) a voltage control signal from the current control section 104 and (ii) a current If through the laser diodes LD1 to LDn for the case in which the laser diodes LD1 to LDn output a light output P1 during an ON period T1 and output a light output P2 (P1<P2) during an ON period T2.
(a) of FIG. 9 illustrates a waveform of the current control signal outputted by the current control section 104 (that is, the gate voltage Vg of the current driving element 103). Since the fiber laser device has a light output that is proportional to the value of a current through the laser diodes LD1 to LDn, the current control signal has a voltage that is higher during the ON period T2 than during the ON period T1.
(b) of FIG. 9 illustrates a waveform of the current If through the laser diodes LD1 to LDn. Since a voltage Vg2 applied to the gate terminal during the ON period T2 is higher than a voltage Vg1 applied to the gate terminal during the ON period T1, a current If2 during the ON period T2 is larger than a current If1 during the ON period T1. Thus, as illustrated in (c) of FIG. 9, a forward voltage Vf2a of the laser diodes LD1 to LDn during the ON period T2 is larger than a forward voltage Vf1a of the laser diodes LD1 to LDn during ON period T1. Similarly, as illustrated in (d) of FIG. 9, a voltage drop Rall×If2 at the resistance Rall during the ON period T2 is larger than a voltage drop Rall×If1 at the resistance Rall during the ON period T1.
(e) of FIG. 9 illustrates a waveform of the output voltage Vcc of the DC power supply 102. The DC power supply 102, which is a constant voltage source, has a constant voltage value Vcc0. Thus, the current driving element 103 has a voltage Vds across its controlled terminals (that is, the voltage across the drain and the source; hereinafter referred to as “inter-terminal voltage”) which voltage Vds is different between the ON period T1 and the ON period T2.
Specifically,Vds1a=Vcc0−Vf1a−Rall×If1,where Vds1a represents an inter-terminal voltage of the current driving element 103 during the ON period T1. Further,Vds2a=Vcc1−Vf2a−Rall×If2,where Vds2a represents an inter-terminal voltage of the current driving element 103 during the ON period T2.
Since (i) Vf1a<Vf2a, (ii) Rall×If1<Rall×If2, and (iii) the output voltage Vcc of the DC power supply 102 is constant,Vds1a>Vds2a unless the current If is 0 (see (f) of FIG. 9). As described above, the laser diode driving device 101 is arranged such that a change in the current through the laser diodes LD1 to LDn changes the inter-terminal voltage of the current driving element 103.
The above arrangement results in a large difference between (i) power consumption Vds2a×If2 by the current driving element 103 during the ON period T2 and (ii) power consumption Vds1a×If1 by the current driving element 103 during the ON period T1 (see (g) of FIG. 9). Large power consumption by a current driving element may break down that current driving element due to a temperature rise. Further, since the current driving element 103 is incapable of controlling the amount of a current through the laser diodes in a non-saturation region, the inter-terminal voltage Vds needs to have a value equal to or greater than a value that allows the current driving element 103 to operate in a saturation region.
Patent Literature 3, in contrast, discloses an invention that (i) includes a variable power supply as a power supply for driving laser diodes, (ii) monitors an inter-terminal voltage of a current driving element, and (iii) controls output of the variable power supply so that the inter-terminal voltage being monitored is equal to a predetermined voltage value. This arrangement allows the inter-terminal voltage of the current driving element to be constant regardless of the amount of a current through the laser diodes. The above arrangement can thus (i) reduce the width of variation in power consumption by the current driving element, (ii) reduce power consumption by the current driving element, and (iii) prevent the current driving element from generating heat.